1. Field of the Invention
The present invention relates to a SAW duplexer including first and second SAW filters having different passbands, and more particularly, to a SAW duplexer in which first and second SAW filters each have a ladder-shaped circuit configuration and a bridging inductor is connected in parallel to a serial arm resonator in at least one of the SAW filters.
2. Description of the Related Art
In recent mobile telephones or the like, SAW duplexers including first and second SAW filters having different passbands are widely used. Japanese Unexamined Patent Application Publication No. 2003-332885 discloses an example of such a SAW duplexer.
FIG. 16 shows a circuit configuration of a SAW duplexer described in Japanese Unexamined Patent Application Publication No. 2003-332885. A SAW duplexer 101 is connected to an antenna 102. That is, a first SAW filter 111 and a second SAW filter 112 are connected to the antenna 102. The first SAW filter 111 and the second SAW filter 112 are different in passband. Each of the SAW filters 111 and 112 has a ladder-shaped circuit configuration including serial arm resonators and parallel arm resonators.
That is, the first SAW filter 111 includes serial arm resonators S1 to S3 and parallel arm resonators P1 and P2. Similarly, the second SAW filter 112 includes serial arm resonators S4 to S6 and parallel arm resonators P3 and P4.
Bridging inductors L1 and L2 are connected in parallel to the serial arm resonators S3 and S5, respectively. The bridging inductor L1 is provided to obtain a sufficient attenuation in the passband of the opposite SAW filter 112. Similarly, the bridging inductor L2 is inserted to obtain a sufficient attenuation in the passband of the opposite SAW filter 111.
In the SAW duplexer 101, a surface acoustic wave element chip is mounted on a package substrate. FIG. 17 is a plan view of a surface acoustic wave element chip 113. In the surface acoustic wave element chip 113, the SAW filters 111 and 112 are formed by placing a plurality of illustrated electrodes on a piezoelectric substrate 114.
The bridging inductors L1 and L2 are not provided in the surface acoustic wave element chip 113. That is, coil-shaped electrode patterns for forming the bridging inductors L1 and L2 are provided on an upper surface of a package substrate 115 shown in FIG. 18.
A plurality of electrode lands electrically connected to the surface acoustic wave element chip 113 are provided on the upper surface of the package substrate 115. Electrode lands A1 and A2 of the electrode lands are connected to the bridging inductor L1. Bumps electrically connected to both ends of the serial arm resonator S3 in the surface acoustic wave element chip 113 are joined to the electrode lands A1 and A2, respectively. Similarly, the bridging inductor L2 is electrically connected to electrode lands A3 and A4 on the package substrate 115. The electrode lands A3 and A4 correspond to portions joined to metal bumps that are connected to both ends of the serial arm resonator S5 provided in the surface acoustic wave element chip 113.
That is, in the SAW duplexer 101, the bridging inductors L1 and L2 are obtained by forming coil-shaped electrode patterns on the upper surface of the package substrate 115 on which the surface acoustic wave element chip is mounted.
Japanese Unexamined Patent Application Publication No. 2003-304139 discloses a package for a surface acoustic wave duplexer. In this package for a surface acoustic wave duplexer, phase-matching circuit electrodes are provided to adjust the phase characteristic of a surface acoustic wave filter mounted in the package. In Japanese Unexamined Patent Application Publication No. 2003-304139, a plurality of electrodes shaped like spiral patterns are described as defining the above-described phase-matching circuit electrodes. The spirally patterned electrodes are provided on a plurality of layers, and are electrically connected to each other by a via-hole electrode.
As described above, in the SAW duplexer described in Japanese Unexamined Patent Application Publication No. 2003-332885, the bridging inductors L1 and L2 are connected to increase the attenuation in the passband of the opposite filter in the filter characteristics of the first and second SAW filters 111 and 112. However, the bridging inductors L1 and L2 are provided by forming coil-shaped electrodes on the upper surface of the package substrate on which the surface acoustic wave element chip is mounted. For this reason, as is evident from FIG. 18, it is necessary to prepare, as the package substrate 115, a large substrate that allows coil-shaped electrode patterns to be formed on the upper surface thereof.
The above-described Japanese Unexamined Patent Application Publication No. 2003-304139 discloses the structure in which spirally patterned electrodes are provided as the phase-matching electrodes on a plurality of layers in the SAW duplexer package. However, in Japanese Unexamined Patent Application Publication No. 2003-304139, the spirally patterned electrodes are simply provided as the phase-matching electrodes, but Japanese Unexamined Patent Application Publication No. 2003-304139 does not specifically refer to the bridging inductor L in the SAW duplexer. While the spirally patterned electrodes serving as the phase-matching circuit electrodes are spiral, the characteristic required of the phase-matching electrodes is completely different from the characteristic required of the bridging inductor.
That is, it is preferable that the bridging inductor have a large inductance and be of a lumped-constant type. In contrast, the phase-matching circuit electrode serves to perform phase matching, and shows a distributed-constant behavior. The length of the spirally patterned electrode is defined by the phase change amount. Therefore, Japanese Unexamined Patent Application Publication No. 2003-304139 describes the structure in which the spiral electrodes are provided on a plurality of layers, but does not disclose the electrode structure that can be used as a bridging inductor in the SAW duplexer.